A build-up of spores after harvest can increase disease pressure the next year.

New information emerging from cherry powdery mildew research shows the potential importance of managing the disease in the orchard throughout the season, not just through harvest.

Cherry powdery mildew (Podosphaera clandestina) is a chronic and serious disease in many cherry orchards. High levels of disease can result in the entire crop being rejected for the fresh market.

Although fruit infections are the primary concern, mildew is commonly found on foliage. Infected leaves then serve as sources of inoculum for later fruit infections and are also where the fungus forms structures to survive the dormant season.

Dr. Gary Grove, plant pathologist at Washington State University and director of WSU’s Irrigated Agriculture Research and Extension Center in Prosser, has studied cherry powdery mildew for years, evaluating fungicide efficacy and developing resistance management and control strategies.

Research he conducted last year suggests that cherries may be unlike grapes and hops that develop age-related resistance to the disease as berries and cones mature. Cherries appear to be susceptible to powdery mildew throughout the season in all growth stages but there may be a period of peak susceptibility.

In wine grapes, fungicide sprays long after bloom to ward off fruit infection are unnecessary because berries become resistant to the infection. However, foliar sprays are often still needed to manage the disease on foliage. Grove was hoping to find a similar stage in cherry fruit growth that became resistant to powdery mildew, but no such luck to date.

“It appears that the susceptibility of cherry fruit is nothing like wine grapes, where fruit is susceptible around bloom but a month after bloom is no longer susceptible,” he said.

Season-long control

Grove said that in cherries (like grapes), the casual organism survives the winter as chasmothecia, which become the disease bridge between growing seasons. “With cherries, you’re managing disease epidemics on fruit and foliage. The problem is that after harvest, the mildew keeps increasing on the foliage.”

When spore populations become high enough on foliage, the disease can deplete tree nutrients, reduce photosynthesis, and, more importantly, set up the orchard for worse problems the following year.

For example, if you took no control measures in 2010 because you had only a low incidence of disease, the overwintering chasmothecia will increase the disease potential the following year. “As the years go by, even with sprays, the disease can intensify and the epidemic will start earlier in the season. The potential for severe disease pressure just gets worse,” Grove said.

To demonstrate the importance of season-long control, Grove sprayed one half of a research orchard at Prosser with fungicides up to harvest, leaving the other half untreated.

As expected, disease severity was initially higher in the untreated block, until after harvest. The last fungicide application was made at the end of June; harvest began in early July. But after harvest, things changed significantly in the trial.

“Later in the season, the block treated before the harvest caught up to the nontreated in terms of disease severity,” he said. “Spore concentrations in the treated block continued to increase during July after spraying stopped. The problem is that once we put the sprayer away and quit treating the foliage, the disease can blow up and cause problems the following year.”

New approach

Grove believes a new approach of controlling powdery mildew throughout the season may be warranted. “The need to control disease on the foliage doesn’t end when the fruit is harvested.”

But before he’s ready to make recommendations, several questions need answering. For example, how long does treatment need to be continued? Is it economically feasible?

And, will season-long control eliminate future fungicide sprays?

“A lot of work needs to be done if we are to develop a season-long program,” Grove said, adding that he is leading research to evaluate its efficacy and impacts on resistance management and long-term disease reduction, and to do an economic analysis.

The research is being funded by a Specialty Crop Research Initiative block grant from the Washington State Department of Agriculture.

Strobilurin resistance

Grove also gave an update on his resistance management research. Data from Washington cherry orchards in 2012 that were inoculated in the laboratory with powdery mildew isolates and analyzed for resistance showed some resistance to the strobilurin class of fungicides.

“Strobilurin resistance was not widespread in the state’s orchards, but we did find a few orchards confined to the Mattawa area,” he said. “It’s evidence that as a class, strobilurin resistance is a concern.”

He also recommends growers avoid using DMI fungicides in their alternations, putting them on the shelf for several years. “Then you can bring them back out later and they will be effective.”

Grove shared his latest research results, funded by the Washington Tree Fruit Research Commission and Oregon Sweet Cherry Commission, with growers attending the Cherry Institute meeting in Yakima, Washington. •

To assist Dr. Grove in his research, cherry growers are encouraged to complete a survey on their mildew control field practices at www.tfrec.wsu.edu/pages/surveys.